Single crystal zinc oxide (ZnO) is potentially useful in optic, electronic and optoelectronic applications as a light emitter, detector or power device component. Good high temperature characteristics coupled with high excitonic and biexcitonic oscillator strength make ZnO a promising material for optical applications. It is fluorescent as well as electroluminescent and it exhibits visible and ultraviolet photoconductivity. Its wide bandgap makes it attractive for the development of blue and ultraviolet light-emitting diodes and lasers. Its piezoelectric properties have been exploited to make surface acoustic wave devices. Its crystallographic properties make it suitable as a substrate material for growing GaN and related single crystal films. A process for making p-type ZnO would enable high temperature and high power solid-state electronic devices.
Zinc oxide crystallizes in the Wurtzite structure (hexagonal space group P63mc , a=3.2498, c=5.2066), which makes it a nearly ideal substrate for growing epitaxial GaN films (hexagonal space group P63mc , a=3.189 c=5.185). The lattice mismatch in the a-direction is less than 2% for ZnO, compared to about 18% for c-cut sapphire. The oxygen atoms are arranged in hexagonal close packing and the zinc atoms occupy half of the tetrahedral holes. Thus each Zn atom is coordinated to four O atoms and each O atom is coordinated to four Zn atoms. The c-axis of the crystal is polar with the (+c) direction conventionally described as “Zn” terminated and the (−c) direction described as “O” terminated. It is a wide bandgap (3.37 eV) II-VI semiconductor. It has a direct gap band structure at room temperature and an excitonic binding energy of 60 meV. Heat-treated ZnO, in which vacancies have been removed by annealing at 800° C., has an electrical resistivity as high as 1010 ohm-cm. It is piezoelectric. Its melting point is 1975° C. The thermal conductivity of ZnO is moderately high (60 W/m-K). The vapor pressure of ZnO is low at temperatures below 1100° C. but, above this temperature, it rises rapidly.
Historically, there are four ways to grow Zn oxide single crystals: (1) from the melt, (2) from high temperature flux solution, (3) by vapor transport, and (4) by high-temperature hydrothermal means.
(1) Growth from the melt is difficult because of ZnO's high melting point (1975° C.), its high vapor pressure (Pvap>10 mm-Hg at T>1500° C.) and its tendency to decompose at high temperature. Decomposition at high temperature precluded crystallization from the melt until recently. A proprietary high-pressure process has now been developed by Cermet, Inc. (Atlanta, Ga.) that prevents decomposition and enables crystal growth from the melt in a Bridgman-like arrangement. (See, Nause, J. et al., “Zinc Oxide (ZnO) Substrates”, International Workshop on Zinc Oxide, (Dayton, Ohio, Oct. 7-8, 1999)). Crystal growth rates are typically in the mm/day range. One difficulty with processes that operate at 2000° C. is that it is difficult to find a suitable material to contain the melt. One solution for ZnO is to use a technique in which the ZnO melt is contained within a ZnO crucible that is kept solid by cooling. This process is inherently expensive because of the high temperatures and high pressures employed.
(2) ZnO crystals can be grown at high temperature in vanadium pentoxidephosphorous pentoxide flux solution by cooling from 1300° C. to 900° C. (See, B. M. Wanklyn, “The growth of ZnO crystal from phosphate and vanadate fluxes,” J. Cryst. Growth, 7, 107-108 (1970)); in lead fluoride solution by cooling from 1100° C. to 800° C. (See, J. W. Nielsen et al., “Growth of large single crystals of Zinc Oxide,” J. Phys. Chem., 64(11), 1762-63 (1960)); or by evaporation of KOH flux between 500° C. and 900° C. (See, M. Ushio and et al., “Synthesis of ZnO single crystals by the flux method,” J. Mat. Sci., 28, 218-24 (1993)). In these cases, as with all crystallizations from flux solution, purity and compositional uniformity are serious issues. The highly volatile PbF2 fluxes are notoriously corrosive even toward platinum and present special problems associated with toxicity. Growth rates are in the range of mm/day. These processes are inherently expensive and difficult to control.
(3) Crystal growth by vapor transport (See, Cantwell, G., et al., “Properties of Vapor Grown ZnO”, International Workshop on Zinc Oxide, (Dayton, Ohio, Oct. 7-8, 1999); E. M. Dodson et al., “Vapour Growth of Single-Crystal Zinc Oxide,” J. Mat. Sci., 3, 19-25 (1968); and R. Helbig, “Über die Züchtung von grösseren reinen und dotierten ZnO-Kristtallen aus der Gasphase,” J. Cryst. Growth, 15, 25-31 (1972)) also takes place at high temperatures (1000-1200° C.) where ZnO(s) reacts with H2(g) to form Zn(g) and H2O(g). In this method one establishes a hot zone and a cold zone in the crystallizer. Zn(g) is transported from the hot zone to the cold zone in flowing hydrogen gas where it is reoxidized on either a sapphire or ZnO seed crystal. One problem with crystal growth by vapor transport is that it is difficult to control, and adding dopants to the growing crystal in a controlled manner is especially difficult. Crystal growth rates are typically 1 mm/day. The process is expensive but can produce fairly large, high-quality crystals.
(4) Conventional hydrothermal crystal growth of ZnO (See, R. A. Laudise et al., “Hydrothermal growth of large sound crystals of zinc oxide,” J. Am. Cer. Soc., 47 (1), 9-12, (1964); E. D. Kolb et al., J. Amer. Ceram. Soc., 49 303 (1966); I. P. Kuz'mina et al., “Synthesis of zincite by the hydrothermal method,” in Crystallization Processes under Hydrothermal Conditions, (A. N. Lobachev ed., Studies in Soviet Science; Consultants Bureau, New York-London, 1973); and M. Suscavage et al., “High quality hydrothermal ZnO crystals,” MRS Internet J. Nitride Semicond. Res., 4S1, G3.40 (1999)) occurs at much lower temperatures than are used in melt-growth, flux-growth or vapor-transport crystal growth methods but which are nonetheless high temperatures for a hydrothermal process. The crystals are grown in a highly caustic solution at conditions (T=355° C., P=8200 psig) near the solution's critical point. A high-strength alloy autoclave, which is capable of containing the high-pressure fluid, is required. For example, a mixture of Li2CO3, 4N NaOH and 4N KOH is used as a mineralizer in which ZnO is highly soluble at 355° C. The solubility of ZnO in such conventional basic mineralizers is ‘normal’, i.e. the solubility is higher at higher temperature.
However, the high temperature and caustic nature of the mineralizer requires that the autoclave be lined with a sealed precious metal liner to prevent contamination of the crystal growth solution and to protect the autoclave from corrosion. Platinum or gold is needed to produce high-resistivity ZnO because less expensive silver is not completely inert at crystallization conditions. Typically, a ΔT equal to a 10 to 20° C. temperature differential is used to achieve transport of ZnO and to achieve the supersaturation condition needed for crystal growth. Pieces of sintered ZnO are preferentially dissolved at the bottom of the autoclave, which is maintained at the high temperature (355° C.). The upper part of the autoclave is maintained at the lower temperature (345° C.) and this is where ZnO seed crystals are placed. In this way the nutrient material is dissolved and transported to the growing crystals by thermal convection. Crystal growth rates are typically 0.25 mm/day. The process can produce large, high-quality ZnO crystals.
There remains a need, however, for lower temperature/pressure processes for growing ZnO single crystals.